mathrm (not italic) in lemmas?

There are certain functions like sin, exp, etc. which are supposed to be written with 
non-italic letters even inside formulas.  Usually this works (by using \sin or C-m 
sin).  However, in the Lemma and Proposition environments (which italicize text) these 
get italicized too.  How can I turn that off?  Or is it even supposed to be this way?

Lyx 1.0.2



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